This invention relates to slip sheets for receiving, handling, storing and shipping a unitized load of product and, more particularly, to an improved paperboard slip sheet.
In many applications, product, e.g., cartoned or bagged product, is unitized to achieve economies by receiving, shipping, handling and storing the product in bulk loads. For many years, these unitized loads have been carried on conventional hardwood pallets. The wooden pallet system for handling unitized loads was adopted as the most popular system initially because the wood pallet was low in cost, availability of wood was adequate, and it was easy to handle the product on the wooden pallet with a standard and relatively low cost forklift truck. However, the use of wooden pallets has suffered from a number of disadvantages including high initial investment cost, problems in inventorying, storing, and returning the pallets; high handling and transportation costs in shipping pallets in two directions; the fact that wooden pallets consume a high percentage of available hardwood; high cost of pallet maintenance to keep the pallets in service and high cost of pallet replacement; problems of lost pallets; and high weight and volume which adds significant cost to shipment and space requirements for storage of the pallets. Wooden pallets can also cause damage to the load during handling and storage, for example, by nails and broken boards rupturing packages mounted on the pallets and product overhang and load settling into broken areas deforming the load. Further, damage can also result to the top of the load when another wooden pallet is placed on top of it.
Because of these and a number of other disadvantages of wooden pallets, the slip sheet method of handling unitized loads was adopted in the mid-1950's and its use has continued to expand up to today. A slip sheet is a thin sheet of material, typically a solid paperboard fiber, which is of a length and width generally the size of the unit load. The slip sheet has one or more "lips" or "pull tabs" which extend about three or four inches beyond the load allowing the slip sheet to be gripped and pulled onto the platens of a forklift truck with the aid of a gripper or push-pull attachment mounted to the forklift truck. Typically, the slip sheets are provided with lips or pull tabs on adjacent sides allowing the load to be picked up either from the front or the side for convenience of loading and full utilization of trailer width, although they can be made with up to four lips or tabs.
Solid fiber is the most commonly used material for slip sheets. Fiber slip sheets have been constructed by laminating three or more sheets of kraft paper together with a waterproof adhesive. The thickness of the sheets typically varies from 0.025 to 0.090 inch. This is typically accomplished by varying the thickness of the individual kraft sheets which make up the lamination. The use of slip sheets has contributed significant economies over the use of wooden pallets. Their cost is approximately 1/10 that of a hardwood pallet; and, since they are expendable, they do not require any maintenance, inventory or return. They can thrown away at the end of the first trip or, if desired, reused until worn out. Moreover, the loss of slip sheets is not a problem. Because they take up essentially no room, slip sheets allow more space in the trailer for the product being shipped. Moreover, since relatively lightweight paper instead of heavy hardwood is being shipped, the weight reduction achieved by using slip sheets instead of wooden pallets creates about a twenty-fold savings of freight weight. Still further, the same number of slip sheets can be stored in about 20% of the space required for wood pallets.
In the palletizing or unitizing process, a stack of slip sheets is typically provided, the sheets are sequentially picked up and transferred one by one to a loading area, and the load unitized on the slip sheet for shipment. Typically, the top sheet of the stack of slip sheets is removed from the stack and transferred to the loading area by means of a suction gripping apparatus. This is a device which includes a number of gripping heads to which a suction is applied to the interior thereof. The suction causes the heads to grip the upper surface of the top slip sheet. The gripper with attached slip sheet is then transferred to the loading and/or palletizing area where the suction is released in turn releasing the slip sheet. The gripper is then moved back to the stack of slip sheets to grip the sheet now at the top of the stack, and the process is repeated. The load to be palletized is placed on the sheet which has been transferred and that load is secured such as by stretch wrapping. It along with its underlying slip sheet is then transferred to a storage or transport area. Once removed, the next slip sheet is gripped by the transfer apparatus and transferred to the loading and/or palletizing area. This transfer process from the stack of slip sheets to the loading area continues until the stack is exhausted.
A major problem which has been observed with this method of transferring slip sheets is that the suction force on the top sheet often penetrates one or two or more sheets below the top sheet thus causing the pick-up and transfer of more than just the top sheet. This problem occurs in slip sheets having a thickness on the order of 0.050 inch and becomes more critical as the thickness of the sheet decreases down to, for example, 0.025 inch. It has been found that it is not possible to sufficiently accurately control the suction force to apply just enough gripping force to grip the top sheet but not the sheet below it. As a result, there exists a significant problem in handling and transporting paperboard slip sheets using the suction gripping method of transfer.
Prior art attempts to solve this problem have included coating the slip sheets with an expensive wax emulsion or calendaring material or providing an extra heavy coating of expensive glue between paperboard laminations forming the slip sheets in an effort to decrease the permeability of the sheets to air flow therethrough. This treatment has several drawbacks including the increased cost of the slip sheets due to the use of expensive coating materials and the fact that the coating requires time for the paperboard to dry thus decreasing the efficiency of the manufacturing operation.